The development of urban transportation planning models has accelerated in the past decade, until they are now widely accepted aids to the urban decision process. However, the models have had limited impact upon individual decisions, because of their complexity and size. In the absence of radical new approaches to planning, a methodology is suggested to complement the existing procedures. This methodology, which is referred to as "perturbation", is illustrated by reference to the choice of a future network. Some results are presented for the budgeting of capital for a set of investments.